Still, wanna bet they won't have as many takers as they planned? Part of the intrigue of living on the street is not having to answer to anyone.
They can't be forced to avail themselves of the voicemail.
I have a bit of experience with this. I worked in social services doing employment assistance for some time. The hard core long term homeless are not their base client. You're right... those people don't go to the places where it's offered.
The typical client I helped set up with voice mail is the temporary homeless surfing from couch to couch, or sleeping in their car, and even those who are unemployed and not yet homeless, but have nevertheless had their phone bills go unpaid too long and they were cut off. The phone bill is one of the first things to go when money runs short, but when it comes to finding a job, it's a necessity.
They're offering 350 voice mail boxes in Washington DC. I think there's probably that many takers.